This invention relates to the protection of sensitive thermal control coating surfaces, and is specifically directed to the application of an improved protective coating over a thermal control coating subjected to a space environment.
Temperature control of spacecraft and space structures is based on heat transfer to and from spacecraft through thermal radiators and solar panels since there is no atmosphere to conduct heat. The amount of heat transferred is determined by the surface optical properties of reflectance or solar absorptance and infrared emittance. For surfaces such as the radiators, it is important to absorb as little solar radiation as possible while radiating or emitting as much heat as possible. By selecting structural materials and/or coatings with specific optical properties, the designer can control the spacecraft's temperature.
Several factors can degrade or alter the optical properties of a thermal control coating. These factors include atomic oxygen, ultraviolet radiation and contamination from other spacecraft components and from nearby rocket engine firings. Experience indicates that contamination deposition can significantly decrease the reflectance. A decrease in the reflectance increases the amount of heat absorbed by the surface. This is particularly detrimental to radiators, since their function is to reject heat from the spacecraft.
Contamination due to outgassing of the materials used in the construction of the spacecraft and redeposition on sensitive surfaces can also significantly reduce the reflectance of these surfaces. The largest amount of deposition usually occurs during the first few years of the spacecraft's life. After the first couple of years, the outgassing from most of these materials is reduced and poses minimal further contamination risk. Another major source of contamination is from "water dumps" or berthing of space vehicles that use bipropellant thruster motors. Although contamination from this source can be controlled to some degree by optimizing exit nozzle directions and the spacecraft approach and engine firings, a method of mitigating the contamination effects on thermal control coatings in close proximity to the thrusters is preferred.
Inorganic (ceramic) white paint thermal control coatings are easily damaged, contaminated or the aluminum substrate corroded during ground handling and storage. This degrades the surface optical properties of the inorganic white paint coating, which can only be restored by extensive baking operations to remove contaminants or removal and reapplication of the coating. Cleaning and repairing coatings on earth is generally expensive and can result in schedule delays. On orbit, the coating is easily contaminated by bipropellant plumes and other orbital debris due to the high porosity of the ceramic coating.
The optical properties of silver-backed FEP (fluorinated ethylene-propylene), marketed as silverbacked Teflon.TM., can degrade in the presence of atomic oxygen (AO) and particle radiation. For long-duration spacecraft, silver-backed Teflon.TM. will slowly be eroded away in low earth orbit by AO. The exact amount depends upon the orientation of the surface to the flight (ram) direction. At a 250 mile (400 km) altitude, Teflon normal to the ram direction will erode unacceptably.
Silver-backed quartz tiles have been shown to be very stable in most space environments. The application technique for the silver-backed quartz tiles is difficult and labor intensive. Tiles are small (generally less than 2".times.2"), expensive and must be manually bonded into place, which increases production and schedule costs. Geometric complexities have made automated application of the tiles impractical. Tiles are also very brittle and tend to be damaged by impacts experienced during ground handling. For large radiative surfaces, silver-backed quartz tiles are impractical for use.
Current anodized aluminum does not possess the optical properties of the previously described coatings. Although anodic coatings are durable and corrosion-resistant, they are not optically stable in ultraviolet (UV) and particle radiation.
In U.S. application Ser. No. 07/887,851, filed May 26, 1992, now U.S. Pat. No. 5,296,285, titled High Emittance Low Absorptance Coatings, by H. W. Babel et al and assigned to the same assignee as the present application, there are disclosed improved thermal control coatings in the form of an inorganic topcoat of a white paint on an anodized aluminum surface to achieve a low absorptance with controlled emissivity.
In U.S. application Ser. No. 07/895,667, filed Jun. 9, 1992, now abandoned, titled Protected Optical Coatings, by H. W. Babel et al, and assigned to the same assignee as the present application, there is disclosed application of an organic topcoat such as a polyurethane resin on an inorganic white paint thermal control coating on a metal, e.g. aluminum, substrate, such organic topcoat serving as a protective coat to maintain surface optical properties of the thermal control coating during service in an outer space environment, and provide corrosion protection of the substrate.
However, although such protective organic coatings can be easily applied, usually at ambient temperatures, the intense U.V. radiation encountered in space causes such protective coatings to darken to various degrees. It has proven very difficult to obtain an organic coating that is easily applied and is also sufficiently UV resistant.
Accordingly, one object of the invention is to provide improved protection of the optical properties of thermal control coatings from contamination during exposure and service in the space environment.
Another object is the provision of a protective coating over a thermal control coating, particularly those with porous surfaces, on a substrate, which is highly resistant to darkening from ultraviolet radiation.
Another object is the provision of a protective coating over a thermal control coating, particularly those with porous surfaces, on a substrate, which protective coating is a slowly eroded coating providing a steady contamination removal as the coating is eroded until the major sources of contamination such as material outgassing and redeposition from other materials on the spacecraft are reduced.
Still another object is to provide a protective coating of the above type which is strongly adherent to the thermal control coating in space and does not significantly affect the optical properties of the thermal control coating.
A still further object is the provision of a protective coating over the thermal control coating which protects the latter coating during cleaning and handling, and also protects the coated substrate from corrosion.
Yet another object is to provide a protective coating of the above type over an inorganic white paint thermal control coating on a bare or anodized aluminum or aluminum alloy substrate.
A still further object is the provision of a protective coating over the thermal control coating which provides additional handling ruggedness compared to thermal control coatings without a topcoat.
Still another object is to provide procedure for application of such protective coating over a thermal control coating on a metal substrate, such as a bare or anodized aluminum or an alloy thereof.
Further objects and advantages of the invention will appear hereinafter.